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Forest Tent Caterpillars

Forest tent caterpillars (FTC) are a common native defoliator found throughout much of Minnesota, especially in the north and central areas of the state. When fully grown, this caterpillar is about two inches long with a blue and black body and white footprint or keyhole shaped spots on their back. FTC feed on the leaves of trees and shrubs, especially aspen, birch, oak, and linden/basswood. Populations of FTC are cyclical, with periods of few and increasing numbers of FTC lasting about 8 – 13 years. Eventually these increasing numbers hit outbreak numbers which lasts about three to four years.

Healthy, mature trees can tolerate severe defoliation, even in several consecutive years. Young and unhealthy trees are more susceptible to injury and should be monitored closely for the potential need to treat. There are a variety of residual insecticides that you can use if you want to protect your plants. Consider using products that have a low impact on the environment, such as Bacillus thuringiensis, spinosad, and insecticidal soap. Bacillus thuringiensis is a particularly good product if the tree is flowering as it will not harm visiting honey bees.

Action Items

Although FTC numbers were down in 2013, if you have experienced them recently, be prepared to treat them if numbers are abundant enough.

The best time to treat them is when they have recently hatched and are small, sometime from early to late May (this will vary depending on where you are in the state and the spring weather).